Optical discs such as DVD-RAMS are phase-change discs including amorphous marks which have been formed on the recording film by directing laser light thereto and controlling the laser power during heating for changing the cooling ratio of the recording film. In order to increase the information transfer rate during recording and reproducing onto and from these optical disc mediums, the recording linear density may be increased or the scanning speed of a light spot over the recording medium can be increased. In order to increase the recording linear density, mark lengths and space lengths themselves may be reduced or mark lengths and space lengths may be varied minutely to reduce the time intervals of detection of mark edge positions. However, the method of increasing the recording linear density will cause the problem of the S/N ratio of reproducing signals, thus making it impossible to largely increase the recording linear density.
In the case of reducing the lengths of to-be-recorded marks and spaces in order to increase the recording density, particularly if the space lengths are made small, this will cause heat interferences, that is, heat of the ending end portion of a recorded mark may be transferred through the space region to affect the temperature rise at the starting end portion of the next mark or heat of the starting end portion of a recorded mark may affect the cooling process at the ending end portion of the previously-formed mark. Prior-art recording methods have had the problem that the occurrence of heat interferences causes fluctuations of mark edge positions thereby increasing the error ratio during reproduction.
Furthermore, even when marks and spaces are formed on a disc to have accurate lengths, there may be caused the problem that the detected edge positions of short marks and spaces are deviated from ideal values during reproduction, due to the frequency characteristics of the reproducing optical system which depend on the size of the light spot. Such deviations of detected edges from the ideal values are generally referred to as inter-code interferences. There has been the problem that when the sizes of marks and spaces are smaller than the light spot, significant inter-code interferences are caused to increase jitter during reproduction, thus increasing the error ratio.
Therefore, there have been disclosed methods which drive laser power with binary values and change the positions of the starting end portions of marks depending on the mark lengths and the preceding space lengths of to-be-recorded marks while changing the positions of the ending end portions of marks depending on the mark lengths and the succeeding space lengths of to-be-recorded marks, as shown in Japanese Patent Publication No. 2679596. Thus, the methods compensate the occurrences of heat interferences between marks during high-density recording and inter-code interferences due to the frequency characteristics during reproduction.
Further, there have been disclosed methods which drive laser power with three or more values and change the positions of the starting end portions of marks depending on the mark lengths of to-be-recorded marks while changing the positions of the ending end portions of marks depending on the mark lengths of to-be-recorded marks during recording, as shown in Japanese Patent Laid-open Publication No. 2004-185796. Thus, the methods compensate the occurrences of heat interferences between marks during high-density recording and inter-code interferences due to the frequency characteristics during reproduction There is also disclosed a method of adjusting the ending end positions of marks by changing the widths of cooling pulses, in such cases.
FIGS. 13A to 13D and 13F are views illustrating examples of marks and spaces in a recording code row and the recording waveform generating operation for recording them, in a prior-art apparatus.
FIG. 13A represents reference-time signals 128 having a period of Tw, which serve as a time reference for the recording operation. FIG. 13B represents a recording code row 126 resulted from the NRZI conversion of to-be-recorded data by the coder 113. Here, the Tw is also a detecting window width and is a standard unit of mark lengths and space lengths in the recording code row 126. FIG. 13C represents an image of marks and spaces to be actually recorded on the optical disc and the laser light spot is scanned in a direction from left to right in FIG. 13C. Marks 301 correspond to the “1” level of the recording code row 126 with a one-to-one ratio and are formed to have lengths corresponding to the durations thereof. FIG. 13D represents count signals 205 for measuring the time elapsed since the heads of the marks 301 and the spaces 302 by using the Tw as a unit.
FIG. 13F is an example of recording waveforms in a prior-art apparatus corresponding to the recording code row of FIG. 13B. The recording waveforms 303 are created by referring to the count signals 205 and the recording code row 126.
FIGS. 14A to 14F are views illustrating examples of marks and spaces in a recording code row and the recording waveform generating operation for recording them, in a prior-art apparatus. FIG. 14A represents reference-time signals 128 having a period of Tw, which serve as a time reference for the recording operation. FIG. 14B represents a recording code row 126 resulted from the NRZI conversion of to-be-recorded data by the coder 113. Here, the Tw is also a detecting window width and is a standard unit of mark lengths and space lengths in the recording code row 126. FIGS. 14C to 14F are timing charts illustrating the waveforms of recording pulse signals 125 during the formation of recording marks having mark lengths of 2 T to 5 T. The recording pulse signals 125 have been subjected to level modulation to have three levels which are a highest-level peak power (Pw), a medium-level erasing power (Pe) and a lowest-level bottom power (Pb) in the case of FIG. 14C.
With the prior-art recording compensation, the amount of shift dTtop by which the starting position of each head pulse is shifted from the reference-time signals is changed depending on the mark length of the to-be-recorded mark as described above, to change the starting end position of the recorded mark. Further, the amount of shift dTe by which the ending position of the cooling pulse is shifted from the reference-time signals is changed depending on the mark length of the to-be-recorded mark to change the ending end position of the recorded mark.
With the aforementioned first prior-art technique, the power is modulated with binary values. Therefore, in the case of performing multi-pulse recording onto a medium such as a phase-change type disc which enables controlling the formation of marks with the cooling rate of heated portions, the next light pulses are directed thereto before the heated portions are sufficiently cooled, which prevents normal mark formation. Namely, there has been the problem that marks are formed to be teardrop shapes and thus normal marks cannot be formed due to excessive amounts of heat injection.
Further, when minute marks have been formed during the mark forming process, marks having minimum mark lengths cause significant inter-code interferences. To cope with this, in order to correct the frequency characteristics of the reproducing optical system, an electrical frequency correcting circuit (equalizer) may be used to reduce the inter-code interferences. However, the boost value of the equalizer is increased especially during the formation of minute marks. When the inter-code interferences in the reproducing system are eliminated by increasing the boost value of the equalizer, noise components in high-frequency regions are increased, thus making it impossible to provide preferable jitter.
Further, with the aforementioned second prior-art technique, the ending end positions of cooling pulses are adjusted during compensation of mark ending edges for facilitating recrystallization of the ending end portions of the mark to adjust the positions of ending end portions of the to-be-recorded mark.
However, in the case of a rerecordable-type optical recording medium employing an inorganic material, the formation of marks have irreversible characteristics and thus includes no recrystallization process of the recording film, which makes it impossible to adjust mark ending end positions by adjusting the widths of cooling pulses, in some mediums. In the case of such mediums, jitter at the mark ending end positions will be increased, thus causing increases of the error ratio of reproducing signals.
As described above, the aforementioned prior-art techniques cannot enable the formation of marks with sufficient accuracy during high-density recording and consequently cannot realize sufficient recording surface densities and sufficient reliability.
Therefore, it is an object of the present invention to provide optical recording methods and optical recording apparatuses which are capable of recording onto various types of optical disc mediums while accurately compensating heat interferences and inter-code interferences.